Measurements of impeller speed, power consumption and gas holdup were combined with visual observations of the extent of gas dispersion and particle suspension in three phase systems for a wide size range of fully baffled, agitated vessels (tank diameters from 0.29 to 1.83 m). Several common impeller geometries were examined and a minimum mixing condition with regard to gas dispersion was specified for each type. Where the impeller had previously been characterized (e. g. the six-bladed disc turbine) in gas-liquid systems, this minimum condition coincided with those generally accepted in the literature. The minimum mixing condition with regard to particle suspension was taken to be when no particle remained at rest on the base for more than one to two seconds. A system was defined as efficiently mixed when both gas dispersion and particle suspension criteria were. simultaneously satisfied for the minimum power input. The effect of well-suspended particles on the gas-liquid hydrodynamics in the vessel was negligible, but if large quantities of particles were settled out on the vessel base, gas dispersion was affected. On the other hand, aeration had an adverse influence on particle suspension, and increased impeller speeds and powers were necessary to maintain the particles in the just-suspended state if the system was sparged with gas. Consequently the result of aerating a system which was operating at the ungassed just-suspended condition was sedimentation of the solids. The severity of this sedimentation was dependent on impeller geometry, and at one extreme could result in complete sedimentation of all the solids for a very small change in operating conditions. A qualitative method is presented which gives an indication of the likely extent of sedimentation. Of the impellers investigated, the disc turbine and mixed flow impeller pumping upwards are shown to provide the most stable and efficient operation at high gas rates. A tentative procedure for designing a disc turbine agitated system is proposed and is supported by data collected over the whole vessel size range. A novel theoretical approach to estimating gas-liquid mass transfer coefficients is utilised to produce data which support the use of disc turbines and mixed flow impellers pumping upwards as the most efficient impellers in a three phase system.